Mixtures and blends of soluble resins with insoluble emulsion polymers are known in the art and are generally used as binders in ink formulations and as overprint coatings to protect printed substrate. The soluble resins were generally prepared by solution polymerization, such as described in U.S. Pat. No. 3,037,952. First generation core-shell resins made significant improvements over mixtures and blends of the prior art, U.S. Pat. No. 4,916,171. By polymerizing one component in the presence of the other to form core-shell compositions, such as described in '171, improvements in production efficiency, in stability, in water resistance, and in rheology were realized. Although the "one-pot" technique of first generation core-shell polymers made significant improvements over the prior art, the instability of first generation core-shell polymers to formulation additives, such as isopropyl alcohol, continued to be a problem for many ink suppliers. The formulation additive problem was overcome by second generation chemically-grafted core-shell polymers.
By using a polyfunctional compound to chemically graft the core to the shell, U.S. Pat. No. 4,876,313, instability of first generation core-shell polymers to formulation additives was resolved. While first and second generation core-shell compositions made significant improvements over prior art mixtures and blends, further improvements in film resistance to alkaline environments were required.
What is required is novel, alkali-resistant, core-shell compositions. These alkali-resistant, core-shell compositions must be resistant to high pH environments such that they could be applied as a clear overprint coating to protect the printed substrate or provide the same protection as an ink vehicle. Neither prior art blends nor first and second generation core-shell compositions are adequately resistant to alkaline environments. Currently, alkali-resistance is required for cereal boxes, detergent boxes, bar-soap wrappers and the like; specifically, and more generally, in applications using conveyor belts or production lines that are lubricated with high pH "line lubricants." For example, alkali-resistance is necessary in order to protect labels on beverage bottles having printed substrates or provide the same protection for the printed label as a clear overprint coating. Therefore, resistance to high pH environments is essential for core-shell polymers to be used in areas where alkaline line lubricants or other alkaline type conditions exist.